This invention relates to nonsegregating dry blends of glass fibers with polymer pellets, suitable for being directly fed to a melt-fabricating machine without being subjected to a preliminary melt-blending step, for producing uniformly loaded fiber-reinforced articles having good mechanical properties such as impact strength and stiffness.
It is customary to blend glass fibers with various polymers, for example, polyamides or polyesters, in order to improve the physical properties of the polymers, especially their toughness and stiffness. Normally, the polymer and the glass fibers are first melt-blended by any one of well known techniques, for example, in a mill, a mixer, or especially a twin-screw extruder. The polymer/glass fiber blend is then pelletized, and the pellets are fed to a melt-fabricating apparatus in which the final article is made. Such precompounding may be undesirable because the polymer is now melt-processed twice, rather then only once, thus being exposed to increased risk of heat degradation. Twin-screw extruder blending produces very uniform dispersions of glass fibers in polymer but has the additional disadvantage of causing a considerable amount of undesirable fiber breakage. A different technique is pultrusion, where a continuous glass fiber is pulled through molten polymer, or through a monomer, which is then subjected to in situ polymerization, or through an oligomer, which is then chain-extended.
If the preliminary melt-blending step is eliminated, and polymer pellets and glass fibers are introduced directly into the feed hopper of a melt-fabricating machine such as, e.g., an injection molding machine, a single screw extruder, or an injection blow-molding machine, the polymer pellets and the glass fibers tend to segregate from each other, because of their different densities, in the feed hopper, the feed throat, and/or the material conveying section of the melt-fabricating equipment and thus tend to form localized polymer aggregates and fiber aggregates. This leads to the production of nominally identical finished articles having different glass fiber levels. Such articles are said to have nonuniform (or variable) fiber loading. Finished articles with nonuniform fiber loading also have nonuniform performance. Nonuniform fiber loading generally causes point-to-point variations in physical properties within parts as well as part-to-part variations. In particular, this can cause inadequate toughening or stiffening at the locations of low fiber concentration and can produce rough, unsatisfactory appearance at the locations of high concentration on the surface of the article.
When a preliminary melt-blending step such as extrusion in a twin-screw machine or pultrusion is added, glass fibers become well dispersed in or coated with polymer. The extruded material or the polymer-coated fiber is cut into pellets that have a consistently uniform glass content. Those pellets can then be melt-fabricated into articles that also have a uniform glass content. While this result is desirable, a disadvantage of melt blending or pultrusion is the additional expense, in addition to the earlier mentioned disadvantages of fiber breakage, usually in an uncontrollable manner, and thermal degradation of the polymer.
It is, therefore, desirable to be able to provide substantially nonsegregating dry blends of glass fibers with particulate polymer material that can be fed directly to a melt-fabricating apparatus, especially to an injection-molding machine or a single screw extruder, without a need to first melt-blend these materials, and to melt-fabricate therefrom articles having a reasonably uniform glass fiber content. It further is desirable to provide nonsegregating, storage-stable dry blends of polymer material with glass fibers that can be transported and fed directly to melt-fabricating equipment.